1. Field of the Invention
The present invention relates to radio apparatus, and it particularly relates to a radio apparatus using multiple subcarriers.
2. Description of the Related Art
An OFDM (Orthogonal Frequency Division Multiplexing) modulation scheme is one of multicarrier communication schemes that can realize the high-speed data transmission and are robust in the multipath environment. This OFDM modulation scheme has been applied to the wireless standards such as IEEE802.11 a/g and HIPERLAN/2. The burst signals in such a wireless LAN are generally transferred via a time-varying channel environment and are also subject to the effect of frequency selective fading. Hence, a receiving apparatus generally carries out the channel estimation dynamically.
In order for the receiving apparatus to carry out the channel estimation, two kinds of known signals are provided within a burst signal. One is the known signal, provided for all carries in the beginning of the burst signal, which is the so-called preamble or training signal. The other one is the known signal, provided for part of carriers in the data area of the burst signal, which is the so-called pilot signal (See Reference (1) in the following Related Art List, for instance).
Related Art List
    (1) Sinem Coleri, Mustafa Ergen, Anuj Puri and Ahmad Bahai, “Channel Estimation Techniques Based on Pilot Arrangement in OFDM Systems”, IEEE Transactions on broadcasting, vol. 48, No. 3, pp. 223-229, September 2002.
In wireless communications, adaptive array antenna technology is one of the technologies to realize the effective utilization of frequency resources. In adaptive array antenna technology, the directional patterns of antennas are controlled by controlling the amplitude and phase of signals, to be processed, in a plurality of antennas, respectively. One of techniques to realize higher data transmission rates by using such an adaptive array antenna technology is the MIMO (Multiple-Input Multiple-Output) system. In this MIMO system, a transmitting apparatus and a receiving apparatus are each equipped with a plurality of antennas, and packet signals to be transmitted in parallel are set (hereinafter, each of data to be transmitted in parallel in a packet signal is called “stream”). That is, streams up to the maximum number of antennas are set for the communications between the transmitting apparatus and the receiving apparatus so as to improve the data transmission rates.
Moreover, combining this MIMO system with the OFDM modulation scheme results in a higher data transmission rate. For the purpose of enhancing the transmission efficiency in this MIMO system, the data signals to be transmitted respectively in a plurality of packets are aggregated into a single packet. In so doing, the control signals are appended to the respective data signals. In other words, a plurality of combinations of control signals and data signals are contained in the packet signals. It is generally the case that the number of subcarries necessary for transmitting the control signal is smaller than the number of subcarriers necessary for transmitting the data signal. Accordingly, if the number of subcarriers used for the transmission of the control signal differs from that used for the transmission of the data signal, the signal strength varies periodically at the time of transmitting packets. That is, the signal strength is attenuated in part of the control signal. When such a fluctuation as this occurs, the signals received by the receiving apparatus also varies. As a result, the power of estimated channel characteristics do not match the power of control signals and thereby the receiving characteristics may possibly deteriorate as will be discussed.
In a MIMO system like this, it is generally the case that the number of subcarries necessary for transmitting the control signal is smaller than the number of subcarriers necessary for transmitting the data signal. The number of subcarriers in the known signal for use in estimating the channel characteristics is made equal to the number of subcarriers in the data signal. If the number of subcarriers used for the transmission of the control signal differs from that used for the transmission of the known signal, the power of estimated channel characteristics do not correspond to the power of control signals and thereby the receiving characteristics may possibly deteriorate as will be discussed.